1. Technical Field of the Invention
The present invention relates to a prism sheet to be used as a display device such as a mobile PC, a PDA, a mobile telephone, a note/desktop PC, a cash dispenser terminal of a bank, or a liquid crystal TV, and a display device using the prism sheet.
2. Description of the Related Art
In the liquid crystal display device of the related art, a diffusion plate and two prism sheets are arranged over a backlight optical guide plate thereby to condense the light from the optical guide plate onto a liquid crystal layer.
The prism sheet is formed by arraying a plurality of prism lenses along one axial direction on one face of a transparent substrate or by making the axial directions of the prism lenses orthogonal to each other on the two faces of a transparent substrate. A various shapes are known as the shapes of those prism lenses such as the regular arrangements of triangular roof shapes, sine-wave shapes or other shapes, or the irregular sizes of the individual prisms. In JP-A-8-262206(hereafter “JPA '206”), the prism sheet, in which concave-convex pattern are formed on the two faces of the surface and the back of a prism sheet and in which the directions of the ridges of the prisms are extended orthogonally of each other on the surface and back of the sheet, is described as one example of the prism sheet of the related art.
A prism sheet 100 disclosed in JPA '206 condenses a light in planes normal to the ridges of the individual prisms of the surface and the back. An example, in which the prism sheet 100 is applied to the display device, is shown in FIG. 13. According to this display device, a light 102 emitted from a light source 101 is reflected on a lamp reflector 103 and fed to an optical guide plate 104. Then, the light extracted from the optical guide plate 104 enters the prism sheet 100 and is refracted and reflected on the lower side prism and further on the upper prism. The light is emitted in the condensed state and then enters a liquid crystal panel 105. Moreover, the individual pixels 106 of the liquid crystal panel are displayed by using the light from the prism sheet 100 as a back light.
In JPA '206, however, the prisms are regularly arrayed to form the Moire fringe (or the interference fringe) easily between themselves and the cells of a liquid crystal or the like, and are defective in the reduction in the display quality. Moreover, the prism lenses are arranged in parallel so that the uneven quantity of light is hard to occur in case the light source is linear or facial. In case, however, the light source has spotted, disc-shaped or spherical shapes, the defect is that the light quantity is liable to become uneven around the position of the light source.